Case Studies in MEMS Case study Technology Transduction PackagingPressure sensor Bulk micromach. Piezoresistive sensing Plastic

+ bipolar circuitry of diaphragm deflection

Accelerometer Surface micromach. Capacitive detection of Metal canproof of mass motion

Electrostatic Surface micromach. Electrostatic torsion of Glass bondedprojection displays + XeF2 release suspended tensile beams

Optical MEMS

Courtesy: H. Toshiyoshi

Why are MEMS used here?

- Structures are the same dimensions as the wavelength - Small displacement has a large effect, can be used for SWITCHING

* Interferometric devices* Scanning devices

- A photon has no mass, easy to deflect light-Can fabricate large-scale systems, (e.g. 1000 X 1000 displays as in the Digital Micro-mirror device)

Applications of Electrostatic projection displays

Courtesy: H. Toshiyoshi

Applications of Electrostatic projection displays

Control of light through:

(1)Reflection : Texas Instruments (DMD: Digital Micromirror Device)

(2) Diffraction: Silicon light Machines (GLV: Grating Light Valve)

Texas Instruments’ Digital Micro-mirror Device (DMD)

- Each rotatable mirror is a pixel - 1024 shades of gray and 35 trillion colors possible- use in projection systems, TV and theaters

The most advanced display technology to date

Distinguishing features of a DMD

• Higher brightness and contrast

• Gray scale achieved by digital and analog modulation- Digital: Pulse Width Modulation (PWM)- Analog: Spatial Light Modulation (SLM)

• Compact, low weight and low power Portable system

H. T

oshiyosh

i

History (1): Si cantilever based light modulator

• SiO2 structural layer• Si sacrificial layer

Petersen, K.E., “Micromechanical light modulator array fabricated on Silicon”, Applied Physics Letters, 31, pp. 521-523, 1977

• Electrically actuated, individually addressable cantilevers• Pull -in

History(2): Torsional electrostatic light modulator

• Bulk micromachining of Silicon

Petersen, K.E., “Silicon torsional scanning mirror”, IBM Journal of Research & Dev., 24, pp. 631-637, 1980

• Electrically actuated torsion mirrors• 1012 cycles, with ± 1o rotation

Elastomer based

History (3): Deformable Mirror DevicesL. Hornbeck, “Deformable Mirror Spatial Light Modulator”,SPIE, vol. 1150, p.86, 1989

Cantilever based

Membrane based

Torsion: Amplitude dependent modulation

Cantilever based: Phase dependent modulation

Digital Micro-mirror device www.dlp.com

DMD Fabrication (6 photomask layers)

Courtesy: H. Toshiyoshi

• Surface micromachining process

• Hinge: Aluminum alloy (Al, Ti, Si) (50-100 nm thick)

• Mirror: Aluminum (200-500 nm thick)

• Aluminum : structural material• DUV hardened photoresist: sacrificial material• Dry release (plasma etching) reduces stiction

DMD superstructure on CMOS circuitry

Texas Instruments DMD characteristics

Digital Micro-mirror device www.dlp.com

Principle of Operation

Balancing electrical torque with mechanical torque

Telectrical is proportional to (voltage)2

Tmechanical is proportional to (deflection

Electrostatic model of a torsion mirror

)sin

(

x)θsinθ

d(rθa

xd

V

a

VE

-Neglect fringing electric field-Neglect any residual stress

Arc length

Electric field

V

x

d

Mirror

Torsion beam

ra

Electrostatic model of a torsion mirror

Electrostatic torque (Telec) = dxx)-

sind

(

xWεV

2

1 Wxdx εE

2

1

2

22

V

x

d

Mirror

Torsion beam

ra

W: widthL: lengtht: thickness

Mechanical torque (Tmech) =

e.g. polysilicon, G = 73 GPag/cm3

Graph Courtesy, M. Wu

Balancing electrical and mechanical Torques

Operation of torsion mirror based DMD

DMD bias cycles

Energy domain model

The torsion mirror as a capacitive device

Calculation of capacitance

From: M. Wu and S. Senturia

From: M. Wu and S. Senturia

Approximate solution- stable angle and pull-in voltage

V

x

dra

V+v

x

dra

V-v

Schemes of Torsion Mirror operation

Pull-in voltage Scan angle Angle-voltageSingle side drive

Push-pull drive

Low Small Non-linear

High Large Linear

V.v α v)(Vv)(V 22

V.v α v)(Vv)(V 22 Bias voltages

Digital Micro-mirror Device (Texas Instruments)

1-DMD chip system

- Can create 1024 shades of gray- used in projectors, TVs and home theater systems

2-DMD chip system

- Can create 16.7 million shades of color- used in projectors, TVs and home theater systems

3-DMD chip system is used for higher resolutions

-For movie projection and other high end applications (35 trillion colors can be generated)

Grating Light Valve (GLV)- Silicon Light Machines (www.siliconlight.com)

Reflection : broad band Diffraction :Wavelength dependent

1 mirror/pixel (2-D array) 6 ribbons/pixel (1-D array)

Larger displacements Displacement: (sec time response) (nanosecond response)

Voltage controlled A fixed angle

Constant intensity Diffracted intensity varied by voltage

Cou

rtes

y: M

.C. W

u

A diffraction grating of 6 beams 1 pixel

Mode of Operation

1 pixel in the GLV: 6 ribbons wide

By using a different spacing between ribbons, one can createdifferent color-oriented pixels

MEMS in Optical Communications

1 X 2 Optical switchOptical fibers

Optical Micro-mirrors used with Add-Drop multiplexers

Bell Labs research

- Very quick switching (> 100 kHz), low losses, - Low cost, batch fabrication

MEMS Micro Optical BenchIntegrable Micro-Optics MEMS Actuators Opto MEMS

Slide courtesy: H. Toshiyoshi

Scratch Drive ActuatorA

kiy

ama,

J. M

EM

S, 2

, 106

, 199

3

- Large total displacements can be achieved (1 mm) @ 100 Hz – 100 KHz- Increments / forward movement as small as 10 nm - voltages required are large

Scratch actuator movement

Voltage applied

MEMS in 3-dimensions“Microfabricated hinges”, K. Pister et al, Sensors & Actuators A, vol. 33, pp. 249-256, 1992

Surface micromachining based

-Assembly of three-dimensional structures- Large vertical resolution and range

H. T

oshiyosh

i

Other variants of the hinge

MEMS in Optical Communications

1 X 2 Optical switchOptical fibers

Optical Micro-mirrors used with Add-Drop multiplexers

Bell Labs research

- Very quick switching (> 100 kHz), low losses, - Low cost, batch fabrication